…g` method

Improve method comments here and there.

…o allow custom logic depending from where key/token value was extracted

# Conflicts:
#	.github/workflows/echo.yml
#	Makefile
#	context.go
#	context_test.go
#	echo.go
#	echo_test.go
#	group.go
#	group_test.go
#	middleware/basic_auth.go
#	middleware/basic_auth_test.go
#	middleware/body_limit_test.go
#	middleware/decompress_test.go
#	middleware/extractor.go
#	middleware/jwt.go
#	middleware/jwt_external_test.go
#	middleware/jwt_test.go
#	middleware/key_auth.go
#	middleware/key_auth_test.go
#	middleware/logger.go
#	middleware/method_override_test.go
#	middleware/recover.go
#	middleware/recover_test.go
#	router.go
#	router_test.go
#	server_test.go